Document EP-A-0 445 002 discloses an installation which includes a conveyor on which trays to be tested are disposed. Prior to sealing the trays, a small quantity of helium is inserted into each one of them. A hood is situated above the conveyor, which hood is connected to an extractor fan via a pipe. The hood can be moved vertically by means of actuators. When the presence of a tray is detected, the hood is lowered, and means secured to the hood exert a certain amount of pressure on the sealed tray. A sniffer probe connected to a helium leak detector is situated at the mouth of the pipe connecting the hood to the fan. If the air flow extracted by the fan contains helium, that indicates that there is a leak in the tray, and control apparatus actuates tray-removal means. Preferably, so as to detect any sealing defects in the bottom of the tray, the conveyor belt is perforated TO enable any gas leaking from the bottom to be entrained by the air flow caused by the fan.
That sniffer system, which operates at a pressure that is very close to atmospheric pressure, requires a certain amount of mechanical pressure to be exerted on the tray so that, in the event that the package is split, a leakage flow is created that is entrained by the air flow. Therefore, that method cannot be used on rigid packages.
When the packages contain fragile foodstuffs, such mechanical pressure exerted on the package may damage the food.
As stated above, the belt must be perforated so as to make it possible to detect any leaks in the bottom, but it may not be enough merely to provide a perforated belt. In the same way, the mechanical pressure, which is exerted by a plate, may prevent a leak from being detected, if the leak is situated in the top of the package under the pressure plate.
The sensitivity of that method is low. The tracer gas driving pressure created in the package by the mechanical means used is not properly controlled. The uniformity of the helium in the air flow created by the fan is not very good.